Semiconductor wafers are subject to a variety of processing steps in the course of the manufacture of semiconductor devices. The processing steps are usually carried out in sealed vacuum chambers of wafer processing machines. Many of the processes performed on the wafers in such chambers involve the heating either of the wafers or of some component involved in the process. As a result of the heating, thermal expansion, including unequal thermal expansion, of parts of the wafer processing machine is encountered.
CVD processes such as those for application of tungsten coatings to semiconductor wafers are typically performed in cold wall reactors, where the waters to be coated are heated to a reaction temperature on a susceptor while other surfaces of the reactor are maintained at subreaction temperatures to prevent the deposition of films thereon. For tungsten CVD, for example, reactor walls are often cooled, often to about room temperature. Alternatively, for titanium nitride CVD, the walls may be heated above room temperature, but to a temperature below that of the substrate being treated.
In a CVD wafer processing machine, the reactor includes a susceptor rotating and wafer elevating mechanism. Evacuation of the reactor is accomplished via a vacuum pump assembly to maintain the reactor at the required operating pressure levels. A resistance heater is mounted in the susceptor for heating the susceptor and hence wafers to the desired operation temperature.
The CVD reactor has a housing which seals within it the reaction chamber. The housing includes provision for independent temperature control, both for heating and cooling of the reactor wall. A mixing chamber is located at the top of the reaction chamber, and reactant gas flows from a showerhead downwardly to blanket the wafers supported on the susceptor.
The semiconductor wafer supporting susceptor provided within the chamber has fixed to its bottom a susceptor drive support frame. Rotatably mounted within the drive support frame is a hollow susceptor drive shaft. The hollow susceptor drive shaft is rigidly connected to the bottom of the susceptor. The hollow space within the drive shaft communicates with the interior of the susceptor within the reaction chamber. The vacuum pressure within the hollow drive shaft is maintained at a pressure sufficiently lower than that of the chamber to develop a vacuum within the susceptor to operate as a vacuum chuck to hold a wafer against the susceptor during processing. Alternatively, if vacuum clamping is not used, the hollow space within the susceptor drive shaft is maintained at a pressure that will develop a vacuum in the susceptor that is equal to or slightly greater than the pressure in the chamber thereby preventing entry of reactant gases into the susceptor.
A seal is therefore provided at the junction or interface between the susceptor drive shaft and the susceptor to prevent fluid communication between the reaction chamber and the interior of the susceptor drive shaft.
The seal between the susceptor drive shaft and the susceptor is subject to thermal cycling during the semiconductor wafer processing steps. For example, during
the production of semiconductor wafers, the susceptor may be heated to approximately 400.degree. to 500.degree. C., at a pressure of from 1 to 100 Torr within the reaction chamber.
At the temperature ranges noted above, metallic seals must ordinarily be used since the operating temperatures exceed the levels at which elastomeric seal materials can survive. Metallic seal materials are inherently less elastic than elastomeric materials, however, and require better control of the gland sealing face separation distance. Unfortunately, thermal cycling often causes the gland sealing face separation distance to change beyond the capacity of the metal seal's elasticity.
The present state of the art relies upon metallic seal assemblies and attempts are made to construct them to have enough elasticity to accommodate the gland seal face separation distance to maintain a leak tight seal. This is sometimes difficult to accomplish, however, because seals must often be small in size leaving very little material with which to provide sufficient elasticity.
Accordingly, there is a need for improvement in sealing between components of wafer processing apparatus which are subjected to thermal cycling at temperatures above acceptable levels for elastomeric materials and which are able to compensate for and thus retain sealing capability during and subsequent to variations in separation distance between gland sealing face surfaces.